The present invention relates to the art of industrial controllers, and more particularly to a method and apparatus for fast FET switching in an output device.
Industrial controllers are special purpose computers used for controlling industrial processes, manufacturing equipment, and other factory automation. In accordance with a control program, the industrial controller measures one or more process variables or inputs reflecting the status of a controlled process, and changes outputs effecting control of the process. The inputs and outputs may be binary, (e.g., on or off), as well as analog inputs and outputs assuming a continuous range of values. The control program may be executed in a series of execution cycles with batch processing capabilities.
The measured inputs received from a controlled process and the outputs transmitted to the process generally pass through one or more input/output (I/O) modules. These I/O modules serve as an electrical interface between the controller and the controlled process, and may be located proximate or remote from the controller. The inputs and outputs are recorded in an I/O table in processor memory. Input values may be asynchronously read from the controlled process by one or more input modules and output values are written directly to the I/O table by the processor for subsequent communication to the process by specialized communications circuitry. An output module may interface directly with a controlled process, by providing an output from an I/O table to an actuator such as a valve, solenoid, and the like.
During execution of the control program, values of the inputs and outputs exchanged with the controlled process pass through the I/O table. The values of inputs in the I/O table are asynchronously updated from the controlled process by dedicated scanning circuitry. This scanning circuitry may communicate with input modules over a bus on a backplane or network communications. The scanning circuitry also asynchronously writes values of the outputs in the I/O table to the controlled process. The output values from the I/O table are then communicated to one or more output modules for interfacing with the process. Thus, the processor may simply access the I/O table rather than needing to communicate directly with the controlled process.
An industrial controller may be customized to a particular process by writing control software that may be stored in the controller""s memory and/or by changing the hardware configuration of the controller to match the control task. Controller hardware configuration is facilitated by separating the industrial controller into a number of control modules, each of which performing a different function. Particular control modules needed for the control task may then be connected together on a common backplane within a rack. The control modules may include processors, power supplies, network communication modules, and I/O modules exchanging input and output signals directly with the controlled process. Data may be exchanged between modules using a backplane communications bus, which may be serial or parallel. A typical hardware modification may involve adding additional I/O modules so as to be able to control additional equipment.
Various control modules of the industrial controller may be spatially distributed along a common communication link in several racks. Certain I/O modules may thus be located in close proximity to a portion of the control equipment, and away from the remainder of the controller. Data is communicated with these remote modules over a common communication link, or network, wherein all modules on the network communicate using a standard communications protocol.
In a typical distributed control system, one or more output modules are provided for interfacing with a process. The outputs derive their control or output values in the form of a message from a master or peer device over a network or a backplane. For example, an output module may receive an output value from a processor, such as a programmable logic controller (PLC), via a communications network or a backplane communications bus. The desired output value is generally sent to the output module in a message, such as an explicit message or an I/O message. The output module receiving such a message will provide a corresponding output (analog or digital) to the controlled process.
Industrial process control systems and devices typically include one or more digital output circuits. Such digital outputs provide binary electrical signals used to interface with one or more components of a controlled process. A digital output may be used, for example, to switch electrical power (e.g., AC or DC) for such applications as energizing actuators, valves, motors, and the like. The switched power may be provided by the control device (e.g., a sourcing output) or externally by a user (e.g., a sinking output). Some digital outputs take the form of a relay contact (e.g., dry contact), which a user may employ to switch external power. Others may include one or more semiconductor switching devices, for example field effect transistors (FETs).
Electrical isolation is sometimes desirable in an output device. In order to achieve isolation of internal controller power from field power, some previous output devices have included an isolation transformer (e.g., a pulse transformer) in the digital output circuitry. However, the use of such a transformer may cause unacceptable electromagnetic interference (EMI) or radio frequency interference (RFI) emissions in an industrial control device. In the field of industrial controllers, reducing EMI and RFI emissions improves overall system safety where some devices in the system (or devices proximate the system) may be susceptible to such interference.
In addition to EMI considerations, the switching time of an output device may be important in certain control applications. In general, better process control is achievable by a controller where the switching time of output devices is reduced. Isolation transformers, such as are commonly used in digital output circuitry, have a finite switching time associated therewith. Thus, the use of an isolation transformer adds to the switching time of other devices in an output circuit (e.g., semiconductor switching devices) when a total device switching time is considered. In addition, isolation transformers increase the EMI/RFI emissions of such circuits during switching, due to the inductances of the transformer primary and secondary windings.
Other conventional industrial control digital output devices have included an optical coupling device (e.g., opto-coupler) to provide electrical isolation between control device power and field power. Although conventional optically coupled digital output devices have reduced the EMI/RFI emissions in industrial controls, switching methods and apparatus are desirable to provide further reduction in switching times, while providing low EMI/RFI emissions and electrical isolation, in order to improve control of industrial processes.
In accordance with the present invention, an industrial control output device is provided including electrical isolation between system power and field power, which minimizes or reduces EMI/RFI emissions therefrom and provides fast switching times (e.g., turn-on time and turn-off time) in selectively providing power to a load. The invention further contemplates a method of providing power to a load in an industrial control system which allows fast switching of a digital output with minimal or reduced EMI/RFI emissions.
According to one aspect of the present invention, there is provided an industrial control device comprising an output with a switching component adapted to selectively provide electrical power from a power source to a load, a first isolation component in electrical communication with the switching component and adapted to selectively energize the switching component, and a second isolation component in electrical communication with the switching component and adapted to selectively de-energize the switching component. According to another aspect, the control device may further comprise a logic component in electrical communication with the first and second isolation components and adapted to provide a signal thereto. The first isolation component may be adapted to selectively energize the switching component according to the signal and the second isolation component may be adapted to selectively de-energize the switching component according to the signal. In addition, the first and second isolation components are adapted to provide electrical isolation between the logic component and the power source, and may further comprise optical coupling devices.
According to another aspect of the invention, the isolation components may comprise opto-couplers, and the switching component may comprise a field-effect transistor (FET) having a gate in electrical connection with the first and second opto-couplers, a source in electrical connection with the power source, and a drain in electrical communication with the load. The first opto-coupler may accordingly be adapted to selectively provide a voltage between the gate and the source of the FET according to the signal, and the second opto-coupler may be adapted to selectively remove the voltage between the gate and the source of the FET according to the signal.
According to another aspect of the invention, the logic component may provide separate signals to the first and second isolation components. Thus, the industrial control device may further comprise a logic component in electrical communication with the first and second isolation components and adapted to provide a first signal to the first isolation component and a second signal to the second isolation component, wherein the first isolation component is adapted to selectively energize the switching component according to the first signal and the second isolation component is adapted to selectively de-energize the switching component according to the second signal. In this regard, the first isolation component may be adapted to selectively provide a voltage between the gate and the source of the FET according to the first signal, and the second isolation component may be adapted to selectively remove the voltage between the gate and the source of the FET according to the second signal.
According to yet another aspect of the present invention, there is provided an output device comprising a switching component adapted to selectively provide electrical power from a power source to a load, a first isolation component in electrical communication with the switching component and adapted to selectively energize the switching component, and a second isolation component in electrical communication with the switching component and adapted to selectively de-energize the switching component. Another aspect of the invention provides for the output device further comprising a logic component in electrical communication with the first and second isolation components and adapted to provide a signal thereto. The first isolation component may accordingly be adapted to selectively energize the switching component according to the signal and the second isolation component may be adapted to selectively de-energize the switching component according to the signal.
According to a further aspect of the invention, the output device comprises a logic component in electrical communication with the first and second isolation components and adapted to provide a first signal to the first isolation component and a second signal to the second isolation component. The first isolation component may be adapted to selectively energize the switching component according to the first signal and the second isolation component may be adapted to selectively de-energize the switching component according to the second signal. The switching component may comprise a FET having a gate in electrical communication with the first and second isolation components, a source in electrical communication with the power source, and a drain in electrical communication with the load, wherein the first isolation component may be adapted to selectively provide a voltage between the gate and the source of the FET according to the first signal, and the second isolation component may be adapted to selectively change the voltage between the gate and the source of the FET according to the second signal.
According to another aspect of the present invention, a method of providing power to a load in an industrial control system is provided. A switching component is provided in electrical communication with a power source and a load, and having a first state and a second state, and electrical power is provided from the power source to the load when the switching component is in the first state. The method further comprises discontinuing power from the power source to the load when the switching component is in the second state, providing a first optical component in electrical communication with the switching component and adapted to selectively energize the switching component, providing a second optical component in electrical communication with the switching component and adapted to selectively de-energize the switching component, selectively placing the switching component in the first state by energizing the switching component using the first optical component, and selectively placing the switching component in the second state by de-energizing the switching component using the second optical component.
According to yet another aspect of the invention, energizing the switching component using the first optical component in the method comprises providing a first signal to the first optical component, whereby the first optical component energizes the switching component, and wherein de-energizing the switching component using the second optical component comprises providing a second signal to the second optical component, whereby the second optical component de-energizes the switching component.